Radioactivity And The Geiger Counter – Part 4

Three Geiger-Muller tubes. They look a bit like large electrical fuses. These were all made in Russia during the 1970s. The STS-5 is identical to the American SBM-20. It’s probably the most common Geiger tube – certainly plenty of them on eBay – and it’s the one that I’ll be using for this article.

Note that these tubes detect beta and gamma radiation and do not detect alpha particles. A special Geiger tube with a very thin ‘window’ is needed to let the alpha radiation pass through. Metal/glass tubes easily block all of the alpha radiation. In fact, a piece of kitchen towel will do!

As I’ve already mentioned a Geiger tube needs around 400 volts to work properly. The current required, though, is tiny; about 5 micro-amps. To put that into some context, a LED will draw about 2 milli-amps. That’s 400 times the current drawn by a Geiger tube. This low current makes it very easy to design a battery powered counter. Two problems: you can easily damage the tube with over-voltage and they don’t work below 350 volts.

Some Messy Electronics

Messy stuff. The board measures approx. 3 inches square

Above is my prototype Geiger tube power supply. It’s powered by 3 AAA batteries. Eventually it’ll be all boxed-up and looking professional.

So, what are we looking at? Let’s start with the big yellow square thing in the middle. That’s the inverter transformer. Open up any electronics device that plugs into a mains socket and you’ll see something similar. This converts 4 volts to about 400 volts. In the case of your ipad or phone charger it converts 240 volts to 5 volts – they work both ways.

Above that there are a red dot and a green dot. You may recall from the last part that I made a simple inverter. It used a transistor to turn the DC from a battery into AC. That’s what the transistor marked with the red dot does.

Left to itself that little transistor can generate over 600 volts. Not too good for a Geiger tube.

The green dotted transistor next to it acts as a regulator or brake. If the red transistor is producing too much voltage the green transistor switches it off, and vise versa.

How does the green transistor know when to switch? If you look at the bottom of the circuit board you may be able to see another green dot. This is a small potentiometer, rather like a volume control on a radio set. It sends a sample of the high voltage back to the regulator transistor. It allows you to adjust the output from about 300 volts to over 420 volts.

This has three advantages: 1) It increases the battery life. 2) It protects the Geiger tube and 3) as the batteries run down it holds the output voltage steady.

You’ll also note that there are two integrated circuits – one marked blue the other white. The blue one is an audio amplifier, so that we can hear the ‘clicks’. The white one is a classic 555 timer wired as mono-stable. Its job is to remove noise and clean-up the signal from the Geiger tube.

Okay, here’s a short video of my Geiger circuit in action. Aka, the Doc irradiating himself 😉

Background Radiation And Dosage

The number of counts-per-minute (clicks) of the background radiation in my house from this tube is around 25. With the uranium tile source approximately two inches from the Geiger tube, and a 0.3mm brass sheet in between, the count goes up to about 90cpm. Without the brass it’s about 600cpm.

What do the clicks/count mean and how safe is it?

For this GM-tube (a STS-5/SBM-20) you multiply the number of clicks-per-minute by 0.006, and this gives you something called micro-sievert per hour (uSv/hour). It’s one millionth of a sievert or 0.000001.

It’s an estimate of how much radiation your body is receiving – the dosage. Remember, radiation accumulates in your body over time.

I detect about 25cpm as the background radiation so that’s 0.15uSv/hr. If we multiply that by 8760 (the number of hours in a year) we get 1314uSv/year.

The UK background average is about 2700uSv/year; well within the safe levels of exposure. In the United States it’s about 6000uSv/year.

The sievert and gray are interchangeable as a radiation dosage measurement. You may recall poor six year old Leide, from part one. She received a radiation dose of 6 gray or 6 sievert. That’s six million micro sievert (6,000,000uSv), forty million times the amount of background radiation that I’m receiving per hour as I type this article!

Radiation And The Inverse Square Law

Referring back to the video you can see that I have marked some numbers on a piece of paper: 0,2,4 and 6. These are the inches in distance from the Geiger tube.

The inverse square law formula is: S ~ 1 ÷ Distance2

This means that the energy from a particular source can be proportionally scaled (S) by the reciprocal of the distance squared from that source.

Don’t run off! It’s really easy to understand. Take a look at the figure below. In it I have represented lamps in two dimensional space. Each lamp is emitting light in all directions equally. The circle on the right is twice the radius of the one on the left. Obviously, the light received at the edge of the bigger circle will be less than the light at the smaller. But by how much?

Figure 1

The area of a circle is equal to 3.14 (π) multiplied by the radius squared. Let d1=22 and d2=42, then the ratio of d2/d1=4. Similarly, the ratio of the area a2/a1 also equals 4. What does that mean?

Well, it tells us two things. One, the lamp’s energy at any point on the circumference of the bigger circle is four times less than that of the smaller; it’s spread over four times the area. Two, we only need to know the distances to compute this fact. For example, if the light at the edge of the smaller circle is 12 units bright it will be only 3 units bright at the bigger edge (12÷4).

This was tested by connecting the Geiger counter to my computer and sampling the clicks. Below is an image of the results.

I placed the uranium tiles two inches from the Geiger tube and counted between 45 and 50 clicks over a period of five seconds. Remember that radioactive decay is random, hence the difference. If I move the tiles to four inches from the GM-tube I should be getting a count of about twelve clicks over five seconds.

Sample clicks. Left: 2 inches distance. Right: 4 inches

At six inches distance that gives us two and one quarter times less energy than at four inches, and nine times less than at two inches. This equates to about five counts over five seconds: 12 ÷ 2.25, and 50 ÷ 9.

Conclusion And Radioactive Quackery

I must have been the only child at my school to have stolen science books; particularly on the subject of electronics. One of those books contained a Geiger counter circuit. A simple project, but at the time I couldn’t obtain a Geiger-Muller tube. Finally, with a more modern circuit, it seems to be seeing the light of day.

It still needs a lot of work: a project box, some sort of counter reading/display and the Geiger-Muller tube needs to be housed in a ‘wand’ with a nice curly cable attached – giving it a professional look.

As a final thought, consider the case of Eben Byers. He won the US amateur golf championship in 1906. He also became obsessed with a ‘health drink’ called Radithor. This was a small amount of radium dissolved in water. After drinking around 1400 bottles of this stuff his jaw fell off and his brain disintegrated. He died in the March of 1932 and was buried inside of a lead-lined coffin.

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